Load support devices with retractable wheels known as auxiliary or tag axle assemblies are well known. Load transporting vehicles traveling on public roadways are frequently subject to a variety of state and/or federal regulations which limit the load per tire or per axle of the loaded vehicle. Tag axles allow a vehicle to comply with these regulations by increasing the number of vehicle axles. Auxiliary wheels are lowered from an elevated stowed position to a ground engaging support position. Design and placement of the tag axle assembly must accommodate the normal functioning of the vehicle.
Tag axle assemblies which extend from the rear of a vehicle and are pivoted between ground engaging and elevated positions are known. U.S. Pat. Nos. 4,940,287 to Ritchie and 4,063,779 to Martin et al. disclose extended reach tag axle assemblies which are mounted to the back end of a dump truck. The rearward extension of these devices in their ground engaging positions increases the overall length of the vehicle and decreases its maneuverability. In addition the wheels must be elevated above the dump bed before the vehicle can discharge its load.
Tag axle assemblies which mount beneath the frame of a vehicle are also known. U.S. Pat. No. 4,079,798 to Ferris discloses a pair of independent tag axle assemblies mounted beneath a concrete truck behind the rear wheels. In the ground engaging position the auxiliary wheels extend behind the back end of the concrete truck. U.S. Pat. No. 5,090,495 to Christenson discloses a pair of independent tag axle assemblies mounted entirely beneath the back end of a refuse truck behind the rear truck wheels. The auxiliary wheels pivot from their upper stowed position to the ground engaging position about a pivot point located between the auxiliary wheels and the rear wheels of the truck. Each auxiliary wheel is moved upwardly or downwardly by the levered action of opposed fluid operated springs. The tag axle is located as far as possible behind the rear driving axle to maximize the distance between the frontmost and rearmost axles of the vehicle.
The tag axle assemblies in the above cited prior art are designed for permanent attachment to a vehicle. Detachable tag axle assemblies are also known. Reissue Pat. No. 30,581 to Schneider discloses a removable supplementary support for the reduction of axle loads. The device consists of a pair of lever arms pivotally attached to each other at one end. The free ends of the lever arms are separated by adjustable biasing means. The lower lever arm has a centrally mounted axle with opposed wheels mounted to the axle on either side of the lever arm. The upper lever arm is adapted to be removably attached to a vehicle frame. The device is designed to be mounted at any appropriate location beneath the frame and removed from the vehicle when the auxiliary wheels are not needed. Consequently the separation between the biased ends of lever arms is restricted and the device does not move the wheels to an elevated stowed position. U.S. Pat. No. 3,860,256 to Jackson et al. discloses a compact tag axle assembly for attachment to a vehicle. The auxiliary wheels are raised and lowered by a dual directional hydraulic piston. The auxiliary wheels are located at one end of a leaf spring which pivots about the other end and the hydraulic cylinder is mounted at a location between the pivot and the auxiliary wheel which is closer to the pivot end than the wheel end. The device is mounted in front of the rear tag axle of a vehicle with the pivot end of the leaf spring at the leading edge of the device.
Vehicle towing apparatus, used to convert a standard pickup or flat bed truck to a tow truck, having secondary wheel and axle assemblies to stabilize the truck during towing are also known. U.S. Pat. No. 4,797,058 to Bilas discloses a supplemental axle assembly associated with a wheel lift vehicle towing device mounted in the truck bed of a pickup type truck. The supplemental wheels are mounted on the truck frame behind the rear axle via vertical air springs. The supplemental axle is secured to the truck's rear axle via a pair of pivotally attached trailing arms. U.S. Pat. No. 4,778,333 to Youmans et al. discloses a vehicle towing apparatus designed to be retrofitted to a standard pickup or flat bed truck having hydraulically positioned wheels which may be lowered during towing of an especially heavy load. The wheels are mounted on the towing device which is attached to the rear of the truck.
The prior art tag axle devices are typically designed to extend the auxiliary wheel behind the vehicle or as close to the rear of the vehicle as possible. Positioning of the deployed tag axle assembly in such a configuration can interfere with normal functioning of the vehicle. Tilting bed vehicle carriers are frequently also equipped with wheel lift towing devices and are particularly susceptible to extreme rear end loads while towing a second vehicle in addition to the vehicle carried on the tilting bed. The tilted bed must also contact the ground as close to the rear of the vehicle as possible while loading vehicles onto the tilted bed. Positioning of a tag axle assembly in front of the rear wheels of a vehicle does not provide additional transfer of load to the front axle by levering the back end upward.
Prior art tag axle assemblies with selectively deployable wheels are bulky and difficult to retrofit to an existing vehicle. Deployment of the wheels in such devices can also interfere with the normal functioning of the vehicle.
It is an object of the present invention to provide a compact tag axle assembly which can be mounted entirely beneath an existing vehicle. A further object of the invention is to provide a tag axle assembly which does not interfere with normal functioning of the vehicle when the wheels are deployed.